Pokémon Red, Blue, and Yellow:
Don't fight the Youngster on Route 34, or the Gambler directly south of the Underground Tunnel exit. Keeping either of them ready to fight and a flying Pokémon in your party can allow you to do the Mew Trick later on. Pressing start as you take the step down needed to bring the Gambler onscreen, and choosing to fly to Cerulean confuses the game as to what happens, and your start menu will refuse to work. To fix this, go to the Youngster, and let it walk to you, or the game freezes otherwise. Once you defeat the Youngster, fly to Lavender Town and exit to the right, then close your menu, which opens automatically. A Wild MEW appeared!